Articles composed of materials having refractory characteristics such as hardness and resistance to erosion have many important uses. Representative materials are described in U.S. Pat. No. 2,938,807 to Anderson.
Reaction sintering of .alpha.-silicon carbide and .beta.-silicon carbide has been known for making high temperature components. For example, .beta.-silicon carbide is described as an excellent binder in the above-mentioned patent to Anderson. However, no super-hard crystals are incorporated into the silicon carbide material.
The use of such super-hard crystals as diamond or cubic boron nitride lends superior characteristics, such as hardness, to the materials. Metals have been used to bind diamond crystals as described in U.S. Pat. No. 4,063,909 to Mitchell. Such metal may be, for example, Co, Fe, Ni, Pt, Ti, Cr, Ta and alloys containing one or more of these metals.
The above and other patents in the area of bonding super-hard crystals depend on hot press technology, as for example that described in U.S. Pat. No. 4,124,401 to Lee et al., U.S. Pat. No. 4,167,399 to Lee et al., and U.S. Pat. No. 4,173,614 to Lee et al., all of which patents are assigned to the assignee of the present invention. Reference is also made to U.S. Pat. No. 4,220,455 to St. Pierre, et al., and which is also assigned to the assignee of the present invention. The latter patent discloses a process for making a homogeneous diamond composite throughout an article, wherein individual crystals are coated and silicon is infiltrated into a porous preform indirectly through a wick material. The diamond composite made in accordance with the technique disclosed in U.S. Pat. No. 4,220,455 is expensive and does not readily lend itself to mass production techniques.
Many of the problems associated with the prior art have been overcome by the inventions disclosed in U.S. patent application Ser. Nos. 167,019, now abandoned, and 167,196, now abandoned both filed July 9, 1980, by Dr. John M. Ohno and assigned to the assignee of the present invention, the entire disclosures of which are hereby incorporated by reference. These patents disclose a straightforward technique (hereinafter referred to as the "press and treat" technique) for forming high quality composites which readily lends itself to mass production techniques. Very briefly, the press and treat technique involves the preparation of a first or a crystal dispersion of super-hard crystals such as diamond or cubic boron nitride crystals in carbon black and a second core dispersion carbon black, carbon fiber and filler material. The two dispersions are individually mixed with a small amount of temporary binder such as paraffin to lend a sufficient green strength to the two dispersions upon cold compaction thereof. After compacting the two dispersions together in a desired configuration, the compact is vacuum heated in the presence of silicon to burn off the paraffin and to allow the silicon to infiltrate both dispersions. Upon further heating, and without the need for the constant application of any type of pressure to the insert, the silicon reacts with the carbon black to form a .beta.-silicon carbide and silicon matrix which bonds both dispersions both internally and to each other.
Further refinements to the press and treat technique described above, have been disclosed in co-pending U.S. patent applications Ser. Nos. 286,613 and 226,604, filed July 24, 1981 and Jan. 21, 1981, respectively, both by Dr. John M. Ohno and assigned to the assignee of the present invention, the entire disclosures of which are hereby incorporated by reference. In Ser. No. 286,613, the technique of providing an interface layer between the crystal and core dispersions is disclosed, the interface layer comprising a dispersion of super-hard crystals at a concentration less than that of the first crystal dispersion layer. By providing such interface layer, the cutting insert produced in accordance with Ser. No. 286,613 eliminates drastic changes or gradients in the composition of the cutting insert structure leading to a more rugged construction.
In Ser. No. 226,604, the second crystal dispersion is employed to form a cutting insert having the first crystal dispersion with the high percentage of crystals covering the top portion of the insert, the second dispersion, having the lower concentration of crystals surrounding the sides of the insert. Also disclosed is the use of a plunger having a chamfer for forming the bottom surface of the compacted core dispersion to compensate for warpage of the resulting composite during the sintering process. See FIGS. 6A, 6B, 7A and 7B of Ser. No. 226,604.
The use of multi-layered structures, such as those discussed above with respect to Ser. Nos. 226,604 and 286,613 may tend to cause problems with compaction of the dispersions due to the difference in compressibility between the various dispersions. For example, the first crystal dispersion, having a high concentration of super-hard crystals, is essentially incompressible, while the core dispersion, having no super-hard crystals therein is relatively highly compressible. The interface or second crystal dispersion, having a concentration of super-hard crystals less than that of the first crystal dispersion, has a compressibility intermediate than that of the first crystal dispersion and the core dispersion.
These differences in compressibilities may lead to non-uniform densifications of the composite, which in turn may cause variations in the amount of silicon infiltration and absorption in different parts of the composite. Such non-uniformities of silicon infiltration may result in warpage of the composite during the reaction sintering process.